1. Field of Invention
This invention relates to polymerizable vinyl adhesive or filling compositions that are useful for a variety of adhesive, coating, filling, repair and related applications. More particularly, this invention relates to two-part room-temperature curing polymerizable vinyl adhesive compositions comprising mixtures of free-radical polymerizable monomers and additives that generate heat and undergo expansion and contraction during the polymerization process. The improved compositions comprise mixtures of elastomers, thermoplastic resins, acrylate, methacrylate and styrenic monomers, and polyester or vinyl ester resins that can be applied in large masses or thick cross sections without gassing and void formation from the exothermic cure reaction. It also relates to improvements in the ability of adhesives based on the compositions to bond thermoplastic and thermoset materials, and to bond such materials with a reduced tendency to cause “read-through” in the bonded area. It further relates to improvements in the ability of the compositions to cure with a tack-free surface and low residual odor, especially when formulated to have a long open working time applications involving large parts or assemblies. It still further relates to improvements in the physical properties and adhesive bonding capabilities of the modified compositions.
2. Background
Polymerizable vinyl adhesive compositions that are useful for a variety of adhesive, coating, filling, repair and related applications are well known in the art. Prior art compositions include formulations based on acrylate and methacrylate monomers, styrene monomer and styrene derivatives as well as polyester and vinyl ester resins. The compositions are generally liquids or pastes that polymerize and cure when two separately packaged components, one of which contains a polymerization initiator, generally a peroxide, and the other of which contains a promoter, generally an amine, are mixed just prior to use.
A particularly useful group of polymerizable vinyl compositions comprises mixtures of dissolved or dispersed polymers in acrylate or methacrylate monomers. Such compositions can provide a number of performance benefits for adhesive bonding and related applications, including high bond strength, adhesion to a variety of materials with minimal surface preparation, and rapid curing. Methyl methacrylate is a preferred monomer for these adhesives because it is relatively low in cost and provides high strength properties in formulated compositions. This group of polymerizable compositions is recognized by those skilled in the art as being superior in many respects to those based on polyester resins and vinyl ester resins, particularly in terms of their ductility and adhesion to a variety of material surfaces.
Polyester resins generally contain styrene, which is lower in cost than methyl methacrylate. They are widely used in automobile body fillers, polyester marine putties, and other filling, bonding and repair materials. Polyester/styrene compositions are preferred for applications that emphasize the ability to economically fill large voids and gaps with adequate functional performance rather than those that emphasize physical properties and related performance attributes at a premium cost. Thus, one surprising aspect of this invention is the achievement of improvements in the performance of the inventive compositions through the incorporation of polyester resins.
Many of the benefits provided by the inventive compositions are useful for the non-adhesive applications cited above. However, adhesive applications are among the most demanding of those anticipated for such compositions. For this reason, the discussion and examples that follow, and the inventive improvements therein will emphasize adhesive applications, with the understanding that they can readily be extended to the other applications cited.
The growing acceptance of methacrylate adhesives has extended their use to larger and larger assemblies and applications, which has resulted in more demanding application requirements. For example, large fabricated assemblies require adhesives with longer open time. For adhesive applications, open time, working time, and open working time are interchangeable terms that define the elapsed time between the mixing of the separate adhesive components and the attainment of a degree of polymerization or cure that prevents effective bond formation. At the end of the open working time, the adhesive either attains a very high viscosity, forms a skin on the surface, or both, preventing effective wetting for good bond formation. For other applications, this interval is often referred to as the gel time or pot life for the composition, which is the time after mixing at which it becomes too thick or viscous to continue applying it. Conventional and prior art techniques for increasing the open working time of adhesives by retarding the onset of cure or cure rate of the composition with chemical inhibitors or retarders often introduce unwanted negative factors or compromises in the application or performance characteristics of the compositions.
Another factor in the use of adhesives for bonding large assemblies is the size of the gaps between the bonded components. As the size of the parts to be bonded increases, so generally does the size of the gap between the mating parts. This can be a particular problem with open molded fiberglass structures, which are prevalent in the construction of boats, large vehicular assemblies, architectural structures, bridge decks, and other large structures. When traditional polymerizable methacrylate adhesives are applied in such thick gaps, the exothermic reaction of curing and the volatility of the monomer generally cause gassing and the formation of voids in the adhesive bond which lead to unacceptable bond integrity and part performance. The additive techniques described above for increasing open working time can also be used to reduce the exotherm and gassing problem, but the same negative application and performance characteristics generally result. Another technique, also discussed below, is the use of inert fillers to reduce this exothermic effect. However, such fillers often have a negative effect on the strength and durability of the compositions in adhesive applications.
Yet another factor addressed by the inventive compositions, especially in the assembly of boats and vehicles, is a phenomenon referred to as read-through or print-through. This is an appearance problem that can result when an adhesive is used to bond an inner reinforcement, stiffener, bracket or other component to an outer panel or “skin” that has a smooth or glossy finish. In transportation applications, such surfaces are generally referred to as “class A” surfaces. At the end of the curing process, or during post-curing processes, certain adhesives, especially those that undergo exothermic polymerization and which change dimensions because of expansion and contraction during the exotherm and cooling associated with the curing process, can contribute to the formation of surface irregularities on the outer or “show” surface of the bonded part. The irregularity is generally a depression, outline, distortion or other disturbance of the surface that is visible to the eye and which is aesthetically objectionable. The occurrence and severity of the problem generally increases with the thickness of the bond and the overall mass of adhesive involved. The causes of such appearance problems can be complex, including contributions from the specific nature of the bonded substrates. These include part thickness, the state of cure of thermoset parts when bonded, the thermal conductivity and expansion coefficients of the bonded materials, the properties of coatings applied on the parts, and other factors. Irrespective of what other factors may be involved in the development of read-through or print through, it is generally observed that adhesives with less tendency to exotherm and undergo dimensional changes during the curing process have less tendency to contribute to the phenomenon.
A number of techniques have been used in efforts to overcome this problem. These often involve the addition of materials that effectively reduce the proportion of reactive monomer in order to reduce its contribution to dimensional changes, shrinkage and exotherm. These materials include inert liquid plasticizers, which act as diluents, and fillers, which act as extenders and absorb some of the heat of polymerization. A related technique involves the use of hollow, expanding microspheres that increase the volume of the curing mass and help offset the shrinkage. Yet another technique is the use of thermoplastic polymers that phase separate during the polymerization process and create internal voids in the material that offset shrinkage. However, as in the case of the gassing problem noted above, the exclusive addition of these inert components in quantities that are sufficient to effectively reduce read through are generally detrimental to the performance of the adhesives. However, they may be used to advantage in combination with the improvements of the inventive compositions.
As previously noted, the usual method of addressing the open working time issues and the exothermic gassing problem, both of which are related to the rate of the curing reaction, is to reduce the reactivity of the composition by using smaller proportions of polymerization initiators, selecting less reactive initiating species, adding retarding additives or chain transfer agents, or a combination of these techniques. However, as pointed out in U.S. Pat. No. 5,859,160 referenced below, these techniques can allow other, undesirable competing side reactions such as oxygen inhibition to interfere with effective polymerization and bond formation. The disadvantages of such air inhibition, as noted in U.S. Pat. No. 5,932,638 also referenced below, include weakening of the adhesive bond, increased odor resulting from escaping, unreacted monomer, and problems related to tackiness of the surface of the adhesive. The problem is especially acute when low levels of catalytic species and added retarding agents are used to extend the open working time of methacrylate based adhesive compositions to periods of about 45 minutes to one hour or more. The problem is further exacerbated by low ambient application temperatures that further reduce the cure rate, and may prevent completion of the free-radical curing process.
Another well-known technique for retarding the cure rate and thereby extending the available time for application of polymerizable vinyl compositions, including methacrylate adhesives, is the addition of certain substituted styrene monomers such as α-methyl styrene. When the composition is based on methyl methacrylate, styrene as well as substituted styrenes are effective, as disclosed in U.S. Pat. No. 5,656,345.
U.S. Pat. No. 5,859,160 discloses the styrenic monomer technique in greater detail, but provides no specific references or examples of adhesive applications or properties, or effects of the added styrenic monomers on them. It is claimed that the deceleration of the cure rate occurs without adversely effecting completion of cure and the properties of the curable composition after it has cured. The use of the invention in formulating adhesive compositions is suggested. It is well known to those skilled in the art that the addition of styrenic monomers to certain methacrylate compositions, especially when combined with low levels of catalytic species to extend open time and reduce exothermic gassing, can have a negative effect on the cure behavior of adhesives.
U.S. Pat. No. 6,291,593 discloses methacrylate adhesive compositions that contain a retarding additive to extend the open time and/or reduce the peak exotherm temperature upon curing. Zinc compounds such as zinc chloride are preferred.
U.S. Pat. No. 5,932,638 discloses the use of certain para-halogenated aniline derivatives to overcome the problems associated with poor surface cure of adhesive compositions resulting from air inhibition. Compositions containing up to about 10 percent by weight of unsaturated polyester resin are disclosed. The cited improvement in the surface cure is a reduction in the thickness of the uncured surface layer exposed to air from about 0.025 inch to about 0.002 to about 0.003 inches. However, actual commercial experience has shown that even the lesser amounts of uncured adhesive cited can be sufficient to cause serious lingering odor problems. Such problems can occur when, for example, the incompletely cured surface of a squeezed out bead or “fillet” of adhesive is in a confined area such the stringer grid of a boat. The problem can be further exacerbated when the fillet or other uncured adhesive bond area is trimmed or smoothed with a spatula or other device that smears a thin film of the adhesive against an exposed surface such as the boat hull. The resulting thin film of adhesive is especially susceptible to the effects of air inhibition. Trapped vapors can eventually migrate to the enclosed cabin area of the boat and create an objectionable or unacceptable level of odor in spite of the very low levels that are present. This is because the detectable odor threshold level for methyl methacrylate monomer is about 0.5 parts per million or less.
There is clearly a need for improved adhesive compositions that provide extended open working time, the ability to cure in large, thick masses without gassing, to provide fully cured, tack free surfaces with little or no residual odor resulting from unpolymerized monomer, and to cure with reduced read-through effects on the finished outer surfaces of boats, vehicles, and other appearance sensitive assemblies while maintaining or improving the performance of the cured adhesive.
It has now been discovered that the combination of polyester or vinyl ester resins and certain acrylate or methacrylate adhesive compositions provides these needed improvements. In contrast to the technique of adding specific retarding additives detailed above, which entail a risk of negative effects on adhesive properties, the addition of polyester resins can impart multiple benefits which will become apparent in the discussion that follows.
U.S. Pat. No. 5,932,638 discloses the optional inclusion of from 0 to about 10 percent by weight of a polyester resin in methacrylate compositions.
U.S. Pat. No. 5,859,160, also discloses the optional inclusion of from 0 to about 10 percent by weight of an unsaturated polyester resin in methacrylate adhesive compositions.
The '638 patent and the '160 patent cite U.S. Pat. Nos. 3,321,351, 4,223,115, 4,293,665, and 4,467,071, which disclose the incorporation of unsaturated polyester resins in methacrylate adhesive compositions. As in the references cited above, the '115, '665 and '071 patents disclose the optional inclusion of from 0 to about 10 percent by weight of unsaturated polyester resin. Example IV in the each of the '115 and '665 patents, which claims improvements in metal adhesive bond durability through the addition of phosphate ester materials, includes 3 percent by weight of an unsaturated polyester resin.
In all of the above-cited references, the methacrylate composition contains at least about 10 percent, and generally 15-20 percent or more of a polymeric species to provide toughness in the cured composition. Preferred polymers include polychloroprene, chlorosulfonated polyethylene, mixtures of chlorinated polyethylene with sulfonyl chlorides, polybutadiene, butadiene copolymers, and polyacrylate rubbers. No particular preference is stated for selection among these polymers, whether or not an unsaturated polyester resin is present.
U.S. Pat. No. 3,321,351 discloses compositions containing unsaturated polyester resins, vinyl monomers and their polymerizates (specifically methyl methacrylate and polymers thereof, styrene monomer and polymers thereof), polychloroprene rubber and polyvinyl ethers. The specification generally discloses 10-85% vinyl monomer, 0-50% vinyl polymer, 0-80% unsaturated polyester and 0-40% polyvinyl vinyl ethers. In the examples, however, when methyl methacrylate is included in the compositions, and no neoprene is included, no more than 15 percent polyester resin is included. When both methyl methacrylate and neoprene are present, no more than 1 percent unsaturated polyester is included. In no case does the amount of neoprene exceed 3 percent of the composition.
U.S. Pat. No. 4,548,992 discloses methacrylate adhesive compositions containing a modified carboxyl containing nitrile rubber and an alkali metal or amine salt of an unsaturated polyester resin. The carboxyl containing nitrile rubber is modified by reaction with a methacrylated phosphate ester. The free carboxyl groups of the polyester resin are neutralized by a metal compound, ammonia or an amine to create a modified polyester resin containing an ionic bond. The ionic bond-containing polyester resin is said to promote adhesion to oily metal surfaces and to improve the storage stability of the methacrylate adhesive composition.